Si fuera como dices, igual estaría condicionado por el diámetro del misil, más aún, empeoraría la cosa, porque los receptores no serían del tamaño máximo permitido por el diámetro.
Creo que te basas en este párrafo de Kropp:
"Several innovative design features were employed. For instance the radar receiver's RF (radio frequency) processor is sandwiched into the actual slotted planar array antenna. Advanced thin-ffim microwave hybrid integrated circuits were used for this purpose. Placing the RF processor up against its antenna improves seeker sensitivity and accuracy by eliminating signal losses and delays inherent in conventional antenna-receiver feeds. The incoming X-band signals are heterodyned down (a common technique for converting signal bearing microwaves down to a frequency where a conventional radio receiver may process them) to the intermediate frequency (I F) within this stage and then fed via coaxial cable into the missile's body. This technique eliminates all of the microwave 'plumbing' in the receiver reducing weight, cost and complexity. The only plumbing which remains is the transmitter's output feed which is routed through the gimballed joint, it carries the high power microwave pulses to the antenna"
http://www.ausairpower.net/amraam.html
Pero entiendo que ahí está hablando del procesador de señales, no de la antena, y que está integrado a la misma.